In large-capacity optical discs such as Blu-ray discs (BDs), in order to reduce an optical spot diameter formed on an optical disc, wavelengths of lasers have become shorter, and numerical apertures (NAs) of objective lenses have become larger. For example, the DVD (Digital Versatile Disc) uses a laser with a wavelength of 650 nm and an objective lens with an NA of 0.65; the BD uses a laser with a wavelength of 405 nm and an objective lens with an NA of 0.85. In general, it is known that the variation of the thickness of a protective layer for protecting an information recording layer of an optical disc generates spherical aberration, and an amount of the generated spherical aberration is proportional to approximately the fourth power of the NA and inversely proportional to the wavelength. Thus, the spherical aberration generated during BD reproduction is approximately 6.5 (≈{(0.85/0.6)4}×(650/405)) times as large as that generated during DVD reproduction. Since the BD reproduction is susceptible to the spherical aberration, it uses a spherical aberration correction means for correcting spherical aberration.
In reproduction of an optical disc, a focus servo control for controlling the position of an objective lens in a direction perpendicular to an information recording surface of the optical disc so that the focus position of laser light follows the information recording surface of the optical disc is performed.
It is desirable that an amount of spherical aberration correction by the spherical aberration correction means and a focus balance value (or focus position) in the focus servo control are adjusted to provide good reproduction quality. However, both are adjustments in an optical axis direction, and affect each other. Thus, techniques for properly adjusting both of the amount of spherical aberration correction and focus balance value (or focus position) are proposed.
Patent Document 1 describes a technique for optimally adjusting a focus balance and a spherical aberration correction amount in an optical pickup device including an objective lens for focusing a light beam on an optical disc and a spherical aberration correction mechanism. This technique obtains tracking error signals (TES signals) at a plurality of combinations of the focus balance and spherical aberration correction amount, selects combinations at which the TES signal is not less than a predetermined level, obtains light intensity signals (RF signals) at the selected combinations, and selects a combination of the focus balance and spherical aberration correction amount having the largest signal level of the RF signal.
Patent Document 2 describes a technique for adjusting the position of a movable lens for correcting spherical aberration and a focus balance value so as to maximize the amplitude level of a tracking error signal (TE signal). This technique obtains in advance, from a graph that has a horizontal axis representing one of the position of the movable lens and focus balance value and a vertical axis representing the other and depicts a contour line of the amplitude level of the TE signal, an inclination α of the long axis of the contour line, which is substantially an ellipse, with respect to the horizontal axis; it seeks a condition in which the amplitude level of the TE signal is maximum by changing either one of the position of the movable lens and focus balance value, and then seeks a condition in which the amplitude level of the TE signal is maximum, along an adjustment line having the inclination α and passing through the found position of the movable lens and focus balance value.
Patent Document 3 describes a technique for two-dimensionally seeking a combination of a focus position and a spherical aberration amount at which the jitter value is optimum, by changing the focus position and spherical aberration amount. Specifically, it discloses: a method that alternates between seeking the focus position at which the jitter value is minimum and seeking the spherical aberration amount at which the jitter value is minimum; a method that repeats a process for determining the point having the smallest jitter value among the four vertexes and center of a rectangular in a graph having axes representing the focus position and spherical aberration amount, while reducing the sides of the rectangular; a method that seeks focus position X1 having the minimum jitter value at spherical aberration amount Y1, seeks focus position X2 having the minimum jitter value at spherical aberration amount Y2, and seeks the point having the minimum jitter value on the straight line connecting the point (X1, Y1) and point (X2, Y2); and a method that seeks the point having the minimum jitter value on a straight line with a predetermined slope a, and then seeks the point having the minimum jitter value on the straight line with a slope of (−1(a) passing through the point.
Patent Document 4 describes a technique for adjusting a spherical aberration correction value and a focus bias value to the optimum values. This technique repeats an operation of measuring jitter values at eight points on the outline of a required assumed margin range on a plane having axes representing the spherical aberration correction value and focus bias value and determining the largest of the jitter values as a representative value, while moving the required assumed margin range in a direction having a predetermined inclination A, and then, using the center of the required assumed margin range in which the smallest representative value has been obtained as an initial point, while moving the required assumed margin range in a direction having a predetermined inclination B, repeats the above operation, specifying the center of the required assumed margin range in which the smallest representative value has been obtained.